It is proposed to identify in developing incisor and molar teeth of fetal and neonatal rats sites of nuclear concentration and retention of vitamin D3 metabolites. Thaw-mount autoradiography will be used, a technique developed in our laboratory and previously applied to the localization of 1,25 (OH)2 vitamin D3 in certain pulp cells of rat incisor teeth. Developmental stages, known to be critical for tooth formation, are selected and include day 16, 18 and 20 of fetal life and day 1, 3 and 6 of neonatal life. The vitamin D metabolites 3H 1,25 (OH)2 vitamin D3, 3H 24,25,(OH)2 vitamin D3 and 3H 25 (OH)2 vitamin D3 will be injected. Competition studies with excess of unlabeled metabolite, injected prior to the radioactively labeled compound, will seek to characterize the chemical nature of the nuclear label. Autoradiograms will be evaluated regarding concentration of radioactivity in pulp cells, odontoblasts, ameloblasts and cells of the stratum intermedium and stellate reticulum. The results of the study are expected to contribute to the understanding of effects of vitamin D on tooth formation and will provide a basis for future studies about relationships to concentrations of 45 Ca and calcium binding protein.